Volume reflection holograms have been mass-produced in the past, predominantly in monochromatic form, by companies such as Applied Holographics plc, Third Dimension Ltd., and Du Pont Authentication Systems inc., by a process of contact copying of image-planed master holograms. These master holograms have generally been created by one of two alternative techniques.
One technique involves the production of second-generation (H2) contact masters from redundant first generation (H1) recordings where the recording of subject matter distant to the recording plate enables multiple channels of image information to be recorded in the H1 such that image-switching effects occur when the second generation hologram is viewed by a viewer's eye, effectively through the window of the virtual H1. This technique is commonly used in holography and is similar to the method used to create classical embossed hologram masters. It is referred to as ‘conventional H1/H2 mastering’, or ‘split beam holography’ and follows the published work by Upatnieks and Leith in the 1960's.
A second technique allows for the initial recording of a first generation master hologram especially in a medium such as dichromated gelatine (DCG), whose grainless clarity, low absorption, and high maximum diffraction efficiency capability enable the preparation of a near-field recording with exceptional brightness and a very wide window of view. This type of hologram where a hologram recording of an image from an object or a master hologram is made with the use of only one expanded laser beam, such that the single beam acts both as the reference beam for the hologram and also gives rise to the object beam when reflected back from an object or hologram, is called a ‘Denisyuk’ or ‘single-beam’ hologram. Recent developments in the production of certain pancromatic ultra-fine grain silver halide materials (such as HRT BBPan from Colour Holographic, London) and modern photopolymer film (such as Bayfol HX from Bayer Materials Science, Chempark, Leverkusen, Germany) mean that these materials can rival the performance of DCG.
The first of these techniques leads to bright multi-channel images which have been criticised by some observers for their limited viewing angle, equivalent to a window of the same size as the H1 master, spaced some distance from the final hologram.
The latter method, however, provides a very wide viewing angle since a master, typically of the same size, may be placed in this case very close to the subject matter. Thus the viewing window of this system may be equivalent to almost a complete hemisphere. However this type of master does not possess the capability to provide animation, or image switching, and because light is diffracted into such a wide viewing zone by this technique the image brightness as perceived from any particular viewing position will tend to be less than with the former method of mastering.
It is nonetheless desirable to be able to record multiple different holographic images in a single recording medium, especially for security applications. It is also desirable to be able to record images onto flexible film rather than rigid glass plates for many reasons including cost and ease of mass production. However the flexible nature of film provides some special challenges which can make recording of holographic images, especially multiple images for security purposes, difficult in practice.
One specific problem associated with recording holograms into film as compared with a glass substrate arises because typically the film must be held substantially motionless (say to better than a quarter wavelength) for a relatively long period whilst the hologram is recorded. This is difficult because film is not rigid. We have designed exposure equipment which uses suitable means for silver halide film and for photopolymer film to enable the recoding medium to remain substantially stationary during the laser recording process. A further problem arises because the hologram emulsion tends to appear cloudy, especially when illuminated with light towards the blue end of the spectrum, and this can also make the recording of multiple different holographic images into a single film hologram difficult. The newly developed recording materials mentioned previously have reduced levels of scatter and absorption which reduce the level of difficulty in achieving high diffraction efficiency in the film hologram copies.
The use of reflection volume holograms for individualised identity purposes has been demonstrated by Bundesdrückerei (Berlin, Germany) in the German passport but the essentially monochromatic 2D (planar) image does not demonstrate the full capabilities of the hologram technology. The principle reason for this is that the representation of 3D full colour images is extremely difficult to achieve in “real time”—that is a timescale compatible with the continuous conventional printing of ID documents such as ID cards or passports.
The recording of a sequence of stereographic images of a human facial portrait can be achieved using a range of displaced camera lenses or by employing a moving camera upon a track (dolly rail). We have previously described a technique employing this approach but involving an intermediate mastering stage (our U.S. Ser. No. 13/876,269). However the intermediate mastering stage is complex and time consuming, in particular since chemical processing is also involved. The method of Yano et al U.S. Pat. No. 4,067,638 dates from 1978. is complex and requires special viewing aids.
The “one-step” method of Haines for embossed holography at Simian Technologies Inc in 1988 was the first system to achieve commercial production of high-quality images through a one-step process. This system was built for Simian Technologies, a company Haines started with his daughter, Debby Haines, a computer programmer who developed the software to process the data from the computer to the LCD, and to automate the system. Simian's precision system created small, full-colour rainbow holograms from 3D animation or video data on a photoresist plate. The end results were holograms for large-run, mass production via a casting/embossing process, but the system requires sensitive, complex software control and is designed to produce transmission hologram linear fringe structures. The process is also relatively time consuming in getting the image data into holographic form, and its throughput speed is incompatible with the present commercial requirements.
We describe techniques which overcome these and other difficulties.